1. Field of the Invention
The invention relates to an optical scanning device for scanning a recording layer of a record carrier by means of a scanning beam, which scanning device comprises a focus-control system for bringing the scanning beam into focus substantially coincident with the plane of the recording layer. The focus control system comprises an actuator for moving the focus relative to the plane of the recording layer; a focus-error detector for generating a focus-error signal which, within a specific operating range of the actuator, is indicative of the distance between the focus and the plane of the record carrier; and a control circuit for deriving, depending on the focus-error signal, a first control signal for the actuator. The scanning device further comprises a signal generator for generating a time-variable additional control signal for the actuator in a time interval in which the focus-control system is inoperative, the signal generator comprising an auxiliary circuit for generating a first input signal for integrating means and a circuit for deriving the additional control signal from the output signal produced by the integrating means. The scanning device also comprises means for rendering the focus-control system operative in response to a lock-in detection signal which indicates that the focus actuator has reached its operating range.
2. Description of the Related Art
The invention further relates to an integrated circuit for use in the focus-control system.
A scanning device of the aforesaid type is disclosed in U.S. Pat. No. 4,128,847. In the scanning device described therein the scanning beam is focused on the record carrier via a focusing objective which is movable along its optical axis. When the focus-control system is operative the focusing objective is driven by the actuator, the actuator being controlled in dependence upon the focus-error signal in such a way that the radiation beam remains in focus on the recording layer of the record carrier.
Before the focus-control system is rendered operative an auxiliary circuit generates a periodic rectangular signal, which is converted into a triangular control signal for the actuator by means of an integrator. The focusing objective is then moved towards or away from the record carrier depending on the direction of the edges of the triangular signal.
When the operating range of the focus-control system is reached the integrating means are set to a hold condition, in which the existing integration value is retained. Moreover, the focus-control system is then actuated by closing the control loop so thereafter the scanning beam is maintained in focus on the record carrier. This method of rendering the focus-contro signal operative is referred to as locking-in of the focus control. After locking-in of the focus-control system the operating point of the known scanning device is, in fact, dictated by the integration value then being held.
This means that the instant of locking-in should be defined accurately. Also, for correct control the operating point of the control system should be situated in the middle of the control range. This which enable focus errors in both directions to be compensated for to a maximum extent.
Another drawback of the known device is that the integration value at the output of the integrator must be maintained constant for a long time, namely until the focus-control system is restarted after the record carrier has been changed. This means that the control point is not adapted to changes in those parameters of the playing device or the record carrier which cause the focal distance between the optical head and the record carrier to change. Moreover, in order to preclude an undesirable variation of the control point a substantially drift-free integrator must be used. In practice, this means that in fact only digital integrators are suitable.